Cell selection methods in multi-cell radio communication systems to effectuate handoffs and to determine initial serving cells are known. One method selects the target serving transceiver based on the arrival time of a signal generated by a remote unit. This method chooses the transceiver that was first to receive the signal based on the presumption that the shortest radio frequency (RF) path correlates with the strongest signal. Another method relies on the absolute magnitude of received signal strength (RSS) as measured by a base site's receiver (or transceiver) resulting in a metric known as a Received Signal Strength Indication (RSSI). This method selects the serving transceiver that received the highest RSSI presuming that the "best" transceiver is that transceiver measuring the strongest signal from the remote unit. However, various signal propagation anomalies such as Rayleigh fading, shadowing (due to obstructions such as tall buildings and deep valleys), ducting, over-water paths (reflective), multipath variations, antenna gain differences (transceiver and remote antennas), signal sampling rates, and co-channel interference all tend to inject error into selection methods resulting in the selection of a serving transceiver that is very distant from the remote unit.
Temporarily selecting a distant serving site, although it received a high RSSI from the remote unit, causes excessive handoffs between cells or dropped calls as soon as the remote unit continues to move away from the serving station as may occur within a building or away from a reflective body of water. In the case of microcellular systems, (in-building systems and public communication networks) wherein a cell radius may be on the order of 10 feet instead of several thousand feet, a mobile remote unit may rapidly travel through numerous cells in short periods of time requiring many rapid handoffs.
One cell selection method as described in U.S. Pat. No. 4,481,670 issued Nov. 6, 1984, invented by Thomas A. Freeburg, and assigned to Motorola, Inc., uses RSSI measurements as measured by all base sites receiving the signal; and adjusts these RSSIs using predetermined factors specific to each cell that received the signal (such as terrain and equipment variations) and computes an adjusted RSSI. The adjusted RSSI measurement is computed for each cell (also using RSSIs as received by other cells) using a matrix of the RSSIs and the predetermined factors. The adjusted RSSIs for each cell are then combined. The cell with the largest adjusted RSSI is determined to contain the remote unit. Such a method considers all cells receiving the signals and therefore typically requires more processing time by a control station, such as a switch, to compute the location of the remote unit. In microcellular systems where numerous handoffs must take place in short periods of time, computationally intensive cell selection methods may cause dropped calls due to the delay in handoff time between cells or the selection of a distant cell that only momentarily provided an adequate communication path.
Another method of cell selection designed to reduce the time between handoffs and choose a closer cell, is described in U.S. Pat. No. 4,718,081, invented by Brenig, issued Jan. 5, 1988. This method takes a weighted average of measured field strengths of a candidate serving site and its neighboring base sites (also candidate cells and cells neighboring to the candidate serving site). The weighted average for all candidate cells are compared and the one with the highest weighted average is the selected serving cell. Although this method may afford some correction for choosing distant cells, the inaccuracy of this method is increased by the limited number of neighboring sites from which RSSI's are taken. Such a system does not adequately determine the location of the subscriber unit to reasonably assure that the selected serving cell is the closest cell to the remote unit. Other information not generally used to locate remote units, such as signals received by non-adjacent cells or impaired signals caused by co-channel interference from other remote units, can be used to more accurately locate a remote unit.
There exists a need for a cell selection method for overcoming erroneous cell selection in microcellular and other cellular systems. Such a method must also select a serving base site substantially close in proximity to the remote unit based upon a determination of a remote unit's location wherein the accuracy of selection is not limited to signal characteristics taken only by base sites neighboring to a potential candidate serving site. The signal characteristics used in the selection process should include other information normally discarded to achieve a more accurate location estimation.